The present invention relates to radio frequency identification (RFID) tags.
A tag serves to identify the thing to which it is attached. RFID tags can be attached to items to aid in their identification, speed checkout processing in a retail environment and aid in inventory management. The RFID tag is scanned or “interrogated” using radio frequency electromagnetic waves. Interrogating the RFID tag with radio waves allows the interrogator to be out of direct line-of-sight of the tagged item and to be potentially located at a greater distance from the item than is permitted with optical scanning.
RFID tags can be either active or passive. Active RFID tags carry their own energy source and passive tags derive their energy from the interrogator's radio signal. When a passive RFID tag is in the vicinity of an interrogator, its antenna receives energy from a radio signal broadcast by the interrogator. This energy is rectified and used to power the RFID tag's integrated circuit. After the passive tag's integrated circuit is powered on, it sends its information to the interrogator.
To reduce costs, inexpensive RFID tags generally do not have a conventional radio transmitter; instead, they communicate with a nearby interrogator using a communication technique known as “backscatter propagation.” Backscatter propagation involves modulating the antenna matching impedance of the RFID tag with the information to be sent to the interrogator. Modulating the impedance in this manner causes varying amounts of radio energy to be reflected from the tag's antenna, which are received and demodulated by the interrogator.
While radio interrogation techniques such as backscatter propagation allow an interrogator to identify a group of RFID tags en masse, determining the physical ordering of the tagged items is problematic. Lack of control over the ordering of items on display can be disadvantageous in a retail situation. For example, when selling items with expiration dates, it is desirable to sell items with the earliest expiration date first. Unfortunately, items displayed on hangers tend to be older at the back than at the front. This is because customers tend to pick items from the front of a display when items at the back are less accessible. This effect is further compounded if the stockers adding new stock cannot readily identify the age of the old stock or if they do not take the extra time to rotate the old stock to the front for easier access.
Accordingly, there is a need for a system to precisely determine the ordering of tagged items in a locality using RFID.
Like reference numbers and designations in the various drawings indicate like elements.